1. Field of the Invention
The present invention relates to coupling means for branching optical fibers used in optical communications systems. More particularly, the invention concerns a coupling ring for branching such optical fibers in a star-type optical coupler.
2. Description of the Prior Art
Known star-type couplers are manufactured as follows: a plurality of optical fibers are drawn from quartz or an industrial plastic. The optical fibers are then branched by binding technologies such as heat melting or ultrasonic welding.
FIG. 1 illustrates a known branching method, according to which a pair of optical fibers 1a and 1b is bound at one branching point 2. A number of optical fibers can be bound through this pair-coupling method. However, branching becomes complicated as the number of branchings increases.
FIG. 2 shows a coupling map inside a star-type optical coupler SC which includes four branching points. In this map, a first light input terminal 1 is connected to a first optical fiber 3a, where it becomes branched with a second optical fiber 3b at a first branching point 4a. The first optical fiber 3a is further branched with a third optical fiber 3c at a second branching point 4b. The second optical fiber 3b that is thus coupled with the first optical fiber 3a at the first branching point 4a, is also coupled with a fourth optical fiber 3d at a third branching point 4c. Thus, a light signal entering the first optical fiber 3a from the light input terminal 1 exits from a light signal output terminal 5 of the first optical fiber 3a. At the same time, the same light signal passes through the third optical fiber 3c branched with the first optical fiber 3a at the second branching point 4b. It also passes through the second optical fiber 3b branched with the first optical fiber 3a at the first branching point 4a, and through the fourth optical fiber 3d branched with this second optical fiber 3b at the third branching point 4c. Thus, the light signal is branched into four light output terminals 5-8. Further, the third optical fiber 3c and the fourth optical fiber 3d are branched at the fourth branching point 4d, so that all the four optical fibers 3a to 3d are mutually branched. In the same manner, the light signals entering the other light input terminals 2 to 4 are all branched through the branching points 4a to 4d and exit at light output terminals 5 to 8.
The above system concerns a system having four branching points (quadruple branching system). However, even when only triple branching is desired, it is common practice to employ the quadruple branching system using four optical fibers 3a to 3d based on the above-mentioned principle. One reason for this is that, by setting the same number of branchings for all the optical fibers 3a to 3d, the light intensity at their light output terminals can be maintained at the same level. Another reason is that standardization of the product specifications for star-type optical couplers SC is thus simpler.
For example, in the system shown in FIG. 2, it may happen that only optical fibers 3a to 3c need to be branched, and not the fourth optical fiber 3d. Even in such a case, all the optical fibers 3a to 3d are branched, but light input terminal 4 and light output terminal 8 are left unconnected to external apparatuses.
In this case, the first optical fiber 3a, the second optical fiber 3b, the third optical fiber 3c and the fourth optical fiber 3d each have two branching points, which are respectively: 4a and 4b, 4a and 4c, 4d and 4b, and 4d and 4c. Each of the optical fibers 3a to 3d thus has two branching points, so that the light intensities at the output terminals are kept even. Consequently, the signal quality in optical communications can be maintained constant.
The above-mentioned double branching of each of the optical fibers 3a to 3d may also be called "two-step branching", and the number of steps may be designated as "m" (where m is an integer). Accordingly, the condition for obtaining a constant light-output quality is that the number of optical fibers to be used can be expressed as 2.sup.m, i.e. 2, 4, 8, or 16 fibers, etc.
In the star-type optical coupler SC shown in FIG. 2, light is branched by virtue of a light advancement vector found in optical fibers 3a to 3d. Therefore, the coupler directs unidirectional light signals as indicated by the arrow "P", so that a plurality of light input terminals 1 to 4 are disposed on one side, while corresponding light output terminals 5 to 8 are disposed on the other side. However, this configuration may create some problems in practical use.
FIG. 3 shows a prior art coupling system, in which the star-type optical coupler SC of FIG. 2 is connected to four light communication apparatuses 5a to 5d. In this case, the coupler is connected to the light communication apparatuses through its light input terminals 8a to 8d and its light output terminals 9a to 9d. As can be seen in FIG. 3, optical fiber cables 6a to 6d and 7a to 7d, respectively wired for light input terminals 8a to 8d and light output terminals 9a to 9d, become entangled at the periphery of the star-type optical coupler SC.
The optical fiber cables 6a to 6d and 7a to 7d are set to have a radius of curvature exceeding the minimum flexing radius, i.e. about 5 to 10 mm, above which the flexing of a cable does not increase optical loss. The cable must thus avoid being flexed into a radius smaller than these figures. Accordingly, the optical fiber cables 6a to 6d and 7a to 7d in the vicinity of the star-type optical coupler SC may become intertwined. It may even not be possible to contain them in a housing, which gives rise to aesthetic problems.
Further, the prior art star-type optical coupler SC includes branching points 4a to 4d in accordance with a multi-step structure. As the number of branching steps "m" becomes greater, the number of optical fibers 3a to 3d used inside the star-type optical coupler leaps exponentially. This in turn increases the number of parts necessary and thus increases material costs.